Asbestos is a range of complex silicates. The main forms of asbestos are:
Chrysotile--white asbestos 3MgO.2SiO.sub.2 2H.sub.2 O
Amosite--brown asbestos FeMgSiO.sub.3
Crocidolite--blue asbestos NaFe(SiO.sub.3).sub.2 FeSiO.sub.3 H.sub.2 O
Anthophyllite--(MgFe).sub.7 Si.sub.8 O.sub.22 (OH).sub.2
These materials break down when subjected to temperatures in the range 400.degree. C. to 900.degree. C. The water is lost from the molecule and the fibrous structure is destroyed leaving simpler compounds such as pyroxenes and silicates. As a result of this breakdown the fibrous nature and therefore the toxicity of the material is destroyed. It would therefore have seemed logical, when attempting to design a process to make asbestos-containing waste permanently non-toxic to consider a heat treatment process.
However, before attempting to design such a process it is necessary to establish the requirements which should be fulfilled to provide a minimum specification for an acceptable process. These requirements are as follows:
(1) The process must render asbestos permanently non-toxic;
(2) The process must be intrinsically safe in operation;
(3) The process must be capable of handling all forms of asbestos and all the materials likely to be associated with asbestos and therefore potentially present in the waste material to be processed;
(4) The process must be simple to operate.
Having established these criteria, the use of thermal decomposition was rejected because it was found that it is impossible in practice to design a safe process. Any malfunction of the time/temperature profile of the system, any change in the composition of the material being fed, variation in water content and, more seriously, incorrect operation by the person controlling the system would create a situation where unconverted asbestos would remain the product leaving the process. In these circumstances instead of making the waste safe the material would in fact have been made more dangerous. There is no method of 100% testing a material for the presence of asbestos and the so-called safe product could still contain potentially dangerous quantities of unconverted asbestos.
Consideration might therefore be given to other properties of asbestos to see if a different approach could solve the problem. All forms of asbestos are silicates. The majority of glass compositions are silicates. It should in principle be possible to convert asbestos into glass. However, there were a number of objections to this approach. Asbestos is a high temperature insulator and its fibrous structure makes heat penetration into the bulk of the material difficult.
Also asbestos is known to produce solid inclusions in glass if it accidentally enters a furnace. Such inclusions arise when the asbestos decomposes into products such as pyroxenes having higher melting points than asbestos itself and higher than conventional glass-making temperatures.
Nevertheless, an electrically powered furnace with a submerged throat off-take was thought to form the basis for a safe process.
The use of inter alia asbestos in glass manufacture was proposed by Crossley in U.S. Pat. No. Re. 15,727 of U.S. Pat. No. 1,394,973. He proposed to heat vitreous material and mica or asbestos to a temperature above the melting point of the glass but below the decomposition temperature of the asbestos so that the physical and chemical characteristics of the mica or asbestos were preserved, and temperatures of 650.degree.-900.degree. C. were employed. But insofar as the asbestos was not destroyed and did not become part of the glass matrix, it retained its toxic properties, and glass derived from asbestos according to Crossley's process would not now be regarded as safe to discharge on a waste tip. The vitrification of asbestos was also taught by Santt in French patent specification No. 2398704 but he considered incorrectly that asbestos is destroyed at temperatures above 600.degree. C. and relied on the presence of iron to form a glass or frit. Although some of the mixtures tested by Santt were melted sufficiently to be pourable, it is not clear that complete melting to produce a homogeneous molten glass had taken place. Nor was it clear, before an actual experiment had been carried out, that asbestos could be melted on a practical as opposed to a laboratory scale.